This invention relates to a closure system for a container, comprising a cap and a ring defining a container opening, which are suitable for screw engagement to seal the opening.
More specifically, the cap comprises an essentially circular top wall from which there extends a downward annular skirt having an annular bottom edge and a radially inward face provided with a screw thread. The ring has an annular rim forming the opening of the container, an annular collar projecting out radially as far as a radially outermost face and a screw thread, complementary to the screw thread of the cap located between the collar and the rim of the ring.
Closure systems of this kind are very common, especially in the form of three-start thread caps for closing drinks bottles. These caps are usually supplied with sealing means which engage with the ring. For example, the cap may have an annular lip that contacts the internal upper portion of the ring, commonly termed the inner lip, and/or an outer lip that engages in a sealed manner with the outer periphery of the rim of the ring. With such lip systems the seal would seem to be completely satisfactory, because when the container is subjected to a large pressure difference between itself and the exterior, no liquid is observed to leak out or gas to get in, even after several hours of tests.
However, it has recently been found that under certain conditions, particularly during very long periods of transport during which the containers are exposed to adverse weather conditions, the radially outermost face of the ring can pick up chemical and/or microbiological contamination even into the screw threads. Although no health incident reported to date can be explained by the discovery of this possibility of contamination, it is preferable to limit it.